The present invention relates to optical signal measurement instruments and more particularly to an optical signal measurement instrument having a wide dynamic range optical receiver for measuring optical signals having wide dynamic ranges, such as in optical time domain reflectometers, optical power meters, and the like.
Optical time domain reflectometers (OTDR) are used for examining optical transmission lines, such as fiber optic cables, for losses and breaks that affect the transmission quality of the lines. The OTDR contains a laser or lasers for generating optical pulses that are launched into a fiber under test via an optical coupler or optical switch. During time intervals between pulses, light reflected back from the fiber is coupled through the optical coupler or switch to an optical receiver. The receiver converts the return optical signal into an electrical signal, which is amplified and converted to digital values for storage and additional signal processing. The return optical signal from an optical fiber under test is composed of a Rayleigh backscatter component and possible reflective components. The Rayleigh backscatter is reflected light caused by impurities and minute imperfections in the fiber and in a good fiber has an exponential decay as a function of fiber length. The reflective components are caused by mechanical splices or breaks in the fiber that reflect substantial portions of the outgoing optical pulse.
The dynamic range of an OTDR is generally given as the one-way difference in db between the extrapolated backscatter level at the start of the fiber and the point where the signal to noise ratio equals one. The backscatter dynamic range is a function of the total optical power launched into the fiber under test, the sensitivity of the optical receiver in the OTDR, and the amount of averaging performed on the return optical signal. For example, the optical dynamic range of a TFP2 Optical Time Domain Reflectometer using a FS1300 Laser Plug-in Module, manufactured and sold by Tektronix, Inc. of Wilsonville, Oreg., is given as 29.5 dB for a 1000 meter pulse from a 1310 nm laser source into a single mode optical fiber. In addition to dynamic range as defined by the backscatter component, a larger dynamic range is required of the optical receiver in the OTDR if it is to completely display all of the reflective components in the optical return signal. To cover the full range of scattering and reflections requires an OTDR having an optical receiver with a dynamic range in the order of 50 dB optical or 200 dB electrical.
The telecommunications industry is experimenting with erbium doped fiber amplifiers in nonrepeated transmission systems of over 200 km in length. A paper entitled "250 km Nonrepeated Transmission experiment at 1.8 Gb/s Using LD Pumped Er.sup.3+ -Doped Fibre Amplifiers in IM/Direct Detection System" by Hagimoto et al., in Electronic Letters, Vol. 25, No. 10, May 11, 1989, describes a direct detection, nonrepeated transmission system over a 250 km low loss dispersion shifted single mode fiber. Erbium doped fiber amplifiers, pumped with 1480 nm lasers, were used as a post amplifier for the launched optical pulses into the fiber and as a preamplifier on the receiver end. A signal gain of 12.4 dB was obtained at an average output power level of +12.2 dBm using a 90 meter postamplifer. U.S. Pat. No. 5,013,907 describes the use of laser amplifiers and fiber amplifiers in OTDRs where the fiber amplifier increases the optical output power of the launched pulses into the fiber under test. In addition, the optical return signal from the fiber under test is amplified by the fiber amplifier. Using fiber amplifiers in OTDRs substantially increases the need for a wide dynamic range optical receiver that can accommodate the greater optical dynamic range of OTDRs with laser amplifiers.
An optical receiver in an OTDR has a photosensitive device that converts the optical return signal into an equivalent current signal. The current signal is converted into an equivalent voltage signal, which in most OTDR's is sampled and stored as digital values. The conversion of the current signal to an electrical signal is accomplished using either logarithmic amplifiers or linear amplifiers. Logarithmic amplifiers are used to compress the input current signal, which has an electrical dynamic range as high as 200 dB, into an output having a small dynamic range. An advantage of using logarithmic amplifiers is that close-in reflective components of the optical input signal may be completely converted to a voltage signal.
U.S. Pat. No. 4,507,615 describes a non-linear amplifier system usable in an OTDR. A current signal from a photodetector is coupled to an input terminal of a first logarithmic amplifier stage that includes a diode connected to the input of the amplifier. The diode develops a voltage there across which is a logarithmic function of the input current signal. The first logarithmic amplifier stage amplifies low input signal levels linearly and higher input signal levels non-linearly. The output of the first amplifier stage is coupled to a second logarithmic amplifier, similar to the first stage, by way of an intermediate limiter stage. The limiter and correction stage linearly amplifies the input signal levels corresponding to low input signal levels of the first amplifier stage. The second amplifier stage amplifies non-linearly the input signals corresponding to the low input signal levels of the first amplifier stage. A combining stage combines the output voltage signals of the first and second amplifier stages and the limiter and correction stage. For a range of low input signal levels, the output voltage signal of the combining stage is representative of the output voltage signal of the second amplifier stage. For a range of high input signal levels, the output voltage signal of the combining stage is representative of the output voltage signal of the first and second amplifier stages. For a crossover range of input signal levels, the output voltage signal of the combining stage is representative of the output voltage signal of the second amplifier stage.
U.S. Pat. No. 5,012,140 describes a logarithmic amplifier with gain control having a reduced system noise floor and increased dynamic range for use in an OTDR. The log amplifier uses a diode for logging the input current signal to a transimpedance amplifier with the current to log voltage transfer function gain controlled with feedback from the amplifier to the log diode. The gain of the log response is adjusted by the feedback gain of a feedback amplifier independent of the transimpedance amplifier gain. Further improvement in the dynamic range is realized by compensating for the negative effects of resistive component of the diode using the feedback amplifier in conjunction additional diode circuitry.
U.S. Pat. No. 4,960,989 describes an optical time domain reflectometer having a receiver with selectively controlled gain. The receiver has an amplification block consisting of series connected transimpedance amplifiers. The gain of each amplifier stage is controlled by a photoconductive switch coupled to second amplification and trigger block. The purpose of this configuration is to decrease the gain of the amplification block in the presence of a reflective component in the optical input signal so as not to saturate any of the amplifiers in the amplification block. A splitter is provided to split the optical return signal between the amplification block and the trigger block. The optical signal in the trigger block is converted to a current signal and is amplified by a logarithmic amplifier. The output of the log amplifier is coupled to one input of a trigger comparator, which receives a trigger level signal at a second input from a controller. The trigger comparator outputs a signal to a laser driver amplifier when the log amplifier signal exceeds the trigger level signal. The laser driver amplifier supplies a current to a laser diode, which emits light at 830 nm that is optically coupled to the photoconductive switches in the amplification block.
U.S. Pat. No. 5,123,732 describes a current to voltage converter for an OTDR where a voltage clipping diode is connected between the input port of the current to voltage converter and a fixed potential to limit the maximum value of the input voltage appearing at the input port. The maximum output voltage is the product of the gain of the amplifying means of the converter and the maximum input voltage to the converter as limited by the clipping diode. The use of the clipping diode prevents the saturation of the amplifier in the converter.
Offsetting the advantages of using logarithmic amplifiers in OTDR's are the disadvantages of design, manufacture, and maintain expenses. Advantages of using linear amplifiers in OTDR receivers are ease of design, manufacture, and support. However, a major disadvantage of using linear amplifiers is that they clip the tops of close-in reflective events because they do not have the dynamic range necessary for displaying close-in reflective components of the optical return signal. To achieve the same dynamic range as log amplifiers would require gain switching for the linear amplifier or separate gain stages. This introduces complexity along with the possibility of generating excess noise. The '989 patent is an example of optical receiver having gain switch limiting but, to keep the noise low, the signals are clipped below the 50 dB level.
What is needed is an optical signal measurement instrument, such as an OTDR or the like, having a wide dynamic range for measuring and displaying the full return optical signal without clipping close-in reflective events. Such an optical signal measurement instrument requires an optical receiver having a wide dynamic range capable of accurately converting the optical signal into an electrical signal representative of the optical return signal. In addition, the optical receiver in such an optical signal measurement instrument needs to be capable of accurately reproducing optical signals that have been amplified by fiber amplifiers or the like. Additionally, the optical signal measurement instrument needs an optical receiver that is simple in design and easy to manufacture and support. Further, it would be desirable to have an optical signal measurement instrument where the optical receiver uses linear amplifiers that have better signal to noise ratios than existing logarithmic amplifiers and have less amplifier tail than log amplifiers.